In a radio frequency substrate, in order to minimize size and obtain high density, a substrate is multilayered and uses many strip lines of which radio frequency wiring passes inside the substrate. However, when connecting radio frequency wiring with a circuit component or an external device, a microstrip line is used because it is necessary to expose the radio frequency wiring on the substrate. Accordingly, it is necessary to connect the strip line and the microstrip line.
A structure for connecting a strip line and a microstrip line using a through-hole which has a conductor inside is proposed by JP, P2002-190546A.
FIG. 4A is a top view showing a connection portion which connects a strip line and a microstrip line of a radio frequency multilayer substrate using conventional technology. FIG. 4B is a cross-sectional view along III-III line of FIG. 4A.
In radio frequency multilayer substrate 100, strip line 6 has 1st ground conductor 51, 2nd ground conductor 52, central conductor 50, and dielectric layers 61 and 62 provided among conductors 50, 51 and 52.
Microstrip line 8 has dielectric layer 61, central conductor 54 and 3rd ground conductor 55. Conductors 54 and 55 are formed on both sides of dielectric layer 61. Central conductor 54 of microstrip line 8 and 1st ground conductor 51 of strip line 6 are formed on a surface of dielectric layer 61.
Radio frequency multilayer substrate 100 has further dielectric layer 63 in order to form other wiring. In the area shown in FIG. 4B, conductor layer 53, conductor layer 56, and conductor layer 57 are formed. 2nd ground conductor 52 is connected to conductor layer 53 using via-holes 71 and 72, and 3rd ground conductor 55 is connected to conductor layer 56 and conductor layer 57 by via-holes 73 and 74. Radio frequency multilayer substrate 100 is grounded externally by conductor layers 53 and 57.
Central conductor 50 of strip line 6 and central conductor 54 of microstrip line 8 are electrically connected using through-hole 75 which has a conductor inside a penetration hole formed in dielectric layers 61, 62 and 63.
From the reasons of manufacturing process of radio frequency multilayer substrate 100, unlike via-holes 71, 72, 73 and 74, through-hole 75 is formed to penetrate dielectric layers 61, 62 and 63.
A part of the conductor of through-hole 75 located below central conductor 50 is unnecessary to connect central conductor 50 and central conductor 54. This unnecessary conductor part functions as a stub, and produces unnecessary stray capacitance 76.
The unnecessary stray capacitance creates a standing wave which subsequently degrades VSWR.